This application proposes to compare genetically defined influenza A virus recombinants with respect to their replication and macromolecular synthesis in different cell culture systems and their ability to be transmitted from infected mice. By comparison of recombinant virus which differ from parent viruses or from each other by a single gene it should be possible to determine which genes contribute to particular strain related differences in biological behavior. In addition information should be obtained relating to the possible requirement for host modification of transcription complexes for vRNA synthesis.